As humans, we have limited resources to control ourselves, researchers say; all acts of control draw from one source. So when using this resource in one domain, such as dieting, we’re more likely to run out of it in another domain, like studying hard.

Inzlicht and Gutsell asked participants to suppress their emotions while watching an upsetting movie. The idea was to deplete their resources for selfcontrol. The participants reported their ability to suppress their feelings on a scale from one to nine. Then, they completed a Stroop task, which involves naming the color of printed words (i.e. saying red when reading the word “green” written in red), yet another task that requires selfcontrol.

The researchers found that those who suppressed their emotions performed worse on the task, indicating that they had used up their selfcontrol resources while holding back their tears during the film.

An electroencephalogram (EEG), a recording of electrical activity in the brain, confirmed the results, they said. Normally, when a person deviates from their goals (in this case, wanting to read the word, not the color of the font), increased activity occurs in a part of the brain called the anterior cingulate cortex, which alerts the person that they are offtrack. The researchers found weaker activity in this brain region during the Stroop task in those who had suppressed their feelings. In other words, after engaging in one act of selfcontrol this brain system seems to fail during the next act, they said.

If we are expected to make moral decisions and are going to be held eternally accountable for them, we have a poor mechanism to do it with. Our brains design is such that it is more likely in any given situation that we will make an error in judgment.